1. Field of the Invention
The invention relates to digital information transmission systems and, more particularly, to a digital information transmission system containing a receiver having a single oscillator for demodulating and decompressing digital information.
2. Description of the Background Art
A conventional digital information transmission system contains a data source, a transmitter, a transmission medium, and a receiver. Illustratively, in a digital television system, the data source is a digitized audio-video signal, the transmitter contains a plurality of application encoders (e.g., a video signal encoder (MPEG), an audio signal encoder, and a system control information encoder), a transport encoder for packetizing and multiplexing the encoded signals and a M-ary quadrature amplitude modulation (QAM) modulator. The transmission medium is typically a cable network. The receiver in a digital television system contains a demodulator for demodulating the QAM signal, a transport decoder for depacketizing and demultiplexing the encoded signals, a plurality of application decoders, and a presentation device for displaying the information from the data source to a user, e.g., the presentation device can be a conventional television. Such a receiver, though not currently available to the public, is foreseen to be a "set-top" unit within a system user's home. This form of digital information transmission system is widely known in the art. One example is a digital National Television Standards Committee (NTSC) television system. A draft standard for video and audio coding techniques to be used in a digital television system is disclosed in "Information Technology-Generic Coding of Moving Pictures and Associated Audio", Recommendation H.222.0, ISO/IEC 13818-1. Jun. 10, 1994.
More specifically, a conventional digital television receiver contains a tuner for selecting a channel of information and a demodulator for demodulating an M-ary QAM signal received from the transmission network. The demodulator is sometimes referred to in the art as a network interface module or NIM. The demodulator produces a serial baseband digital signal (a bit stream containing packetized and multiplexed digital information). As is well-known in the art, the demodulator accomplishes carrier recovery, signal equalization, packet synchronization and the like, to generate a useful baseband digital signal. The baseband signal must be further processed by a transport decoder to extract from the baseband signal the video, audio and timing information within the data packets.
The conventional transport decoder in an digital television system generally functions as a sophisticated phase lock loop that operates at 27 MHz, e.g., the transport decoder is synchronized to a 27 MHz reference oscillator within the transmitter that contains the application encoder. This 27 MHz clock rate (known as the transport clock rate) is double the standard video sampling rate used by the transport encoder under standard CCR 601. Since the receiver uses the 27 MHz clock signal only for decoding the encoded information carried in the transmitted packets, the 27 MHz clock signal is completely unrelated to the transmission rate for channel symbols. In other words, the received signals, once demodulated using conventional timing synchronization techniques, produce a baseband digital signal having symbol and bit rates that are unrelated to the 27 MHz. Typically, the symbol rate is much less than the transport decoder clock rate. To synchronize the baseband signal symbol rate to the transport clock signal rate, the baseband symbols are written to a buffer at the symbol rate and read from the buffer at the transport decoder clock rate. To facilitate such buffering, a first-in, first-out (FIFO) memory is used. Detrimentally, since the demodulator and the transport decoder operate at two different frequencies, they each must have their own frequency sources (e.g., separate crystal oscillators for separately timing the write and read FIFO operations).
Additionally, using a FIFO to temporarily store symbols in this manner requires the system to operate at a single symbol rate unless multiple oscillators are available to provide a number of write address rates, i.e., a clock rate commensurate with each symbol rate. In a conventional receiver having a multiple symbol rate capability, when the symbol rate changes, the timing system is altered to accommodate the new symbol rate, i.e., a different clock rate is selected to control the FIFO write operation. Consequently, multiple symbol rate receivers are complicated and costly.
Therefore, a need exists in the art for a receiver that uses a clock signal from a single crystal oscillator to accomplish symbol timing and transport decoder timing. Additionally, a need exists in the art for a receiver that uses a single crystal oscillator and yet demodulates a plurality of symbol rates.